1. Field of the Invention
The present invention relates to a pretreating apparatus for the adhesion of plastic sheets, and more particularly, is directed to a pretreating apparatus which confers a primer treatment and an adhesive application treatment onto areas to be coated with adhesive of plastic sheet having poor adhesive properties preparatory to the adhesion thereof.
2. Description of the Related Arts
Plastic sheets, in particular, transparent or translucent rigid plastic sheets have been often used in the form of packages or cases for accommodating foods or a variety of groceries and packing them. The process of manufacturing the packages or cases generally comprises the steps of previously applying adhesive onto adhesive joints of plastic sheets cut in accordance with a predetermined development, introducing the plastic sheets into an adhesive bonding apparatus to fold them, and pressurizing or heating under pressure the adhesive joints for the adhesion.
Some plastic sheets have good adhesive properties and others present a poor ones. For example, polyvinyl chloride plastic sheets are satisfactory in adhesive properties, whereas sheets made of polyolefin plastic such as polyethylene or polypropylene plastic are extremely poor. It is difficult to directly bond such plastic sheets having poor adhesive properties, and hence their adhesive joints must be subjected to a primer treatment for surface roughening preparatory to the application of the adhesive in a pretreating apparatus.
For use in such primer treatment, for example, available are primer solutions such as Aron primer (pretreatment agent for aron alpha which is an instantaneous adhesive) from Toagosei Chemical Industry Co., Ltd. and Cyano primer from Koatsu Gas Kogyo Co., Ltd. Paper or cloth impregnated with such primer solution is used to wipe the adhesive joints to roughen them.
Such conventional manner of pretreating before adhesion is, however, inefficient and poor in reliability. Particularly, the primer treatment involves the following critical problems.
(i) The treatment with the primer solution is often ununiform, which inevitably leads to unevenness of surface roughening. PA1 (ii) In an extreme case, a portion possibly remains unroughened. PA1 (iii) The applied primer solution may be squeezed out of the adhesive joints, which remarkably impairs the outward appearance when assembled into packages or cased and lowers the product value. PA1 (iv) The treatment is slow in speed. PA1 (v) The automatization of the subsequent adhesive bonding step is difficult to carry out due to the unreliability of the treatment and the slowness of treatment speed.